Method of Production of Ultralow Carbon Cast Slab

ABSTRACT

A method of production of an ultralow carbon cast slab characterized by adding Ti to molten steel decarburized to a carbon concentration of 0.01 mass % or less, further adding at least one of Ce, La, and Nd, and using an immersion nozzle to inject the above molten steel from a tundish to a casting mold for continuous casting while maintaining a flow rate of Ar gas blown from any location in a range from a tundish upper nozzle to discharge ports of said immersion nozzle at 3 Nl (normal liter)/min or less.

TECHNICAL FIELD

The present invention relates to a method of producing an ultralowcarbon cast slab by continuous casting.

BACKGROUND ART

The dissolved oxygen in molten steel refined by a converter or vacuumtreatment container is generally removed by the deoxidizing element Al.However, if deoxidizing ultralow carbon steel with a large content ofdissolved oxygen by Al, alumina (Al₂O₃) will form. These agglomerate andmerge to form large amounts of coarse clusters of hundreds of micronssize or more.

Part of these alumina clusters enter the immersion nozzle from thetundish at the time of continuous casting. If sticking at the inner boreof the immersion nozzle, they cause nozzle clogging and obstructoperation. Further, if the alumina clusters enter the casting mold andremain at the surface layer of the cast slab, they become causes offormation of surface flaws of thin-gauge steel sheet and have adetrimental effect on quality.

As a countermeasure against this, in general there is the method ofblowing in Ar gas from a tundish upper nozzle, sliding nozzle, orimmersion nozzle to cause the Al₂O₃-based inclusions to stick to thesurfaces of bubbles, prevent them from sticking at the inner bore of theimmersion nozzle, and cause them to float up in the casting mold forremoval.

However, with this method, there were the problems that the Ar bubblesblown in became the cause of pinhole defects in the cast slab and that,further, the Ar bubbles floating up in the casting mold disturbed themeniscus and caused powder entrainment, with the entrained powderparticles becoming the cause of surface flaws at the thin-gauge steelsheet.

Further, when continuously casting molten steel, from the viewpoint ofease of production, usually, as shown in FIG. 7, an immersion nozzle 1of a straight shape with a fixed inside diameter from the top end to thebottom end of the inner bore 10 is used.

However, in the case of an immersion nozzle with an inner bore of astraight shape, as shown in FIG. 8, the opening part 12 of the slidingnozzle 11 is offset from the center of the immersion nozzle 1, so whenthe molten steel in the tundish (not shown) passes through the slidingnozzle 11 and flows into the immersion nozzle 1, similarly, as shown inFIG. 8, an uneven distribution of the molten steel flow rate inevitablyoccurs in the immersion nozzle 1 (in the figure, see downward arrows atthe center).

Due to this, there was the problem that uneven flows 13 a, 13 b withdifferent flow rates occurred at the left and right discharge ports, thestate of fluid motion in the casting mold was disturbed, and powder orbubbles were carried to deep positions in the unsolidified part of themolten steel and remained in the cast slab.

To solve these problems, two methods of solution have been disclosedbased on prior discoveries. The first is the method of using animmersion nozzle having an orifice in the inner bore for molten steelkilled by Al.

This method has as its object the prevention of sticking of alumina inthe inner bore and suppression of uneven flow. For example, JapanesePatent Publication (A) No. 2001-239351 discloses an immersion nozzlehaving a plurality of step differences in its inner bore. Further,Japanese Patent Publication (A) No. 2004-255407 discloses an immersionnozzle having, a plurality of discontinuous projections in the innerbore.

Further, in these patent documents, if providing an orifice in the innerbore (step differences or projections), the part in the immersion nozzlewhere the molten steel flow rate becomes remarkably slow is eliminatedand the flow rate is made uniform. As a result, it is disclosed, theeffects of suppression of uneven flow and prevention of sticking ofalumina are obtained.

Further, Japanese Patent Publication (A) No. 2001-239351 discloses thatan inert gas flow rate is suitably 1 Nl (normal liter)/min to 40N1(normal liter)/min. Note that below, “normal liter” will sometimes besimply expressed as “Nl”.

The second is the method of preventing the formation of aluminaclusters. For example, Japanese Patent Publication (A) No. 2002-88412,Japanese Patent Publication (A) No. 2003-49218, Japanese PatentPublication (A) No. 2003-268435, Japanese Patent Publication (A) No.2005-60734, and Japanese Patent Publication (A) No. 2005-139492 disclosethe method of using Ti and rare earth metals for deoxidation(hereinafter referred to as “deoxidation by Ti-rare earth metals”.

This method deoxidizes the molten steel by Ti to form Ti oxides, thenadds rare earth metals to change the Ti oxides to spherical inclusionsresistant to agglomeration and merger. According to this method, it ispossible to prevent sticking of inclusions on the immersion nozzle,clogging of the immersion nozzle, and formation of surface flaws due toalumina clusters.

Further, Japanese Patent Publication (A) No. 11-343516 discloses themethod of adding one or both of Ca and a rare earth metal afterdeoxidation by Ti and continuously casting without blowing in Ar gas.This method as well is a method suppressing the formation ofcluster-like inclusions and making the inclusions finely disperse. Bythis method, it is possible to obtained titanium-killed steel excellentin surface properties.

DISCLOSURE OF THE INVENTION

First, the method disclosed in Japanese Patent Publication (A) No.2001-239351 and Japanese Patent Publication (A) No. 2004-255407, thatis, the method of using an immersion nozzle having an orifice in itsinner bore to continuously cast Al-killed molten steel will beexplained.

In the technology disclosed in the above patent documents, even if usingan immersion nozzle having an orifice in its inner bore, the effect ofmaking the distribution of the flow rate uniform is hard to obtain. Thisis because in the above art, there is the problem of nozzle clogging.

This reason is that at the part below the bottom end of the orifice, aneddy flow occurs and stirring is caused, so the alumina will not stick,but at the part above the top end of the orifice, no eddy flow occurs,so sticking of alumina cannot be avoided.

In particular, the top end of the orifice is the location where stickingof alumina progresses the most. If a large amount of alumina-basedinclusions stick here, the nozzle will clog.

As disclosed in Japanese Patent Publication (A) No. 2001-239351, ifblowing Ar gas into the molten steel, nozzle clogging can be prevented.However, part of the Ar gas blown into the molten steel fills theimmersion nozzle and pushes the melt surface in the immersion nozzle(secondary meniscus) down in position.

The molten steel flowing in from the tundish to the immersion nozzledrops freely from the position of the sliding nozzle to the secondarymeniscus, but if the secondary meniscus is pushed down, the droppingdistance of the molten steel will become longer, so a strong downwardflow will easily occur right below the dropping position of the moltensteel.

In some cases, as a reaction to this, as shown in FIG. 9, a reverserising flow 14 (in the figure, see dotted line arrows) occurs whereby anuneven distribution of the molten steel flow rate occurs in theimmersion nozzle (in the figure, see solid line arrows).

The uneven distribution of the molten steel flow rate occurring due tothe dropping flow of the molten steel is eased if making the secondarymeniscus close to the position of the sliding nozzle. However, to obtainthe effect of prevention of sticking of alumina, at least apredetermined level of Ar gas flow rate is required. Normally, the Argas flow rate is 5 to 20 N1/min, but with this Ar gas flow rate, it isdifficult to make the secondary meniscus close to the position of thesliding nozzle.

If the secondary meniscus is low and the distance to the top end of theorifice (in the figure, see “21 a”) is short, there is the possibilitythat the molten steel will not stop once at the part above the orificebut will pass through the orifice with the distribution of the flow ratestill not made uniform. Accordingly, in the state of a low secondarymeniscus, it is difficult to suppress uneven flow caused by a droppingflow by just the orifice.

Next, the method disclosed in Japanese Patent Publication (A) No.2002-88412, Japanese Patent Publication (A) No. 2003-49218, JapanesePatent Publication (A) No. 2003-268435, Japanese Patent Publication (A)No. 2005-60734, Japanese Patent Publication (A) No. 2005-139492, andJapanese Patent Publication (A) No. 11-343516, that is, the method ofcontinuously casting molten steel not forming any alumina clusters willbe explained.

In the art disclosed in the above patent documents, the inclusions areresistant to agglomeration and merger, so no coarse clusters are formedand the nozzle is resistant to clogging. However, in the above patentdocuments, the shape of the inner bore of the immersion nozzle is notdefined. No technical matter relating to the secondary meniscus isdescribed.

In the above art, no means is devised for suppressing unevendistribution of the flow rate and uneven flow, so there is a highpossibility of powder or bubbles being carried to deep positions of theunsolidified part of the molten steel and of powder or bubbles remainingin the cast slab becoming a cause of surface flaws formed when worked toa thin-gauge steel sheet.

In this way, in the prior art, there was the problem that it wasdifficult to achieve both prevention of nozzle clogging and securing ofthe cast slab quality. Note that the “securing of the cast slab quality”here means stably producing a cast slab free of surface flaws even ifworked to a thin-gauge steel sheet.

The present invention, in consideration of these problems, has as itsobject the provision of a method of production of an ultralow carboncast slab enabling achievement of both efficiency of continuous castingand cast slab quality.

The inventors engaged in repeated research to solve the above-mentionedproblems and as a result discovered that if deoxidizing ultralow carbonmolten steel by Ti-rare earth metals (Ce, La, Nd) and using an immersionnozzle having an orifice in its inner bore, it is possible to preventnozzle clogging and continuously cast an ultralow carbon cast steel freeof surface flaws even if worked to a thin-gauge steel sheet.

The present invention was made based on the above discovery and has asits gist the following constitutions:

(1) A method of production of an ultralow carbon cast slab characterizedby adding Ti to molten steel decarburized to a carbon concentration of0.01 mass % or less, further adding at least one of Ce, La, and Nd, andusing an immersion nozzle to inject the above molten steel from atundish to a casting mold for continuous casting while maintaining aflow rate of Ar gas blown from any location in a range from a tundishupper nozzle to discharge ports of said immersion nozzle at 3 Nl (normalliter)/min or less.

(2) A method of production of an ultralow carbon cast slab as set forthin (1), characterized in that said immersion nozzle has an orifice inits inner bore.

(3) A method of production of an ultralow carbon cast slab as set forthin (2), characterized in that

-   -   said inner bore has a circular cross-sectional shape and    -   (i) a relation 3≦R−r≦30 stands between a radius R of a top end        of the inner bore [mm] and a smallest radius r of the orifice        [mm] and    -   (ii) a length L from a top end to a bottom end of the orifice        [mm] is 50≦L≦150.

(4) A method of production of an ultralow carbon cast slab as set forthin (2), characterized in that

-   -   said inner bore has an elliptical cross-sectional shape and    -   (i) a relation 3≦A−a≦30 stands between a radius A of a long-axis        direction of a top end of the inner bore [mm] and a smallest        radius a of the orifice [mm] and    -   (ii) a length L from a top end to a bottom end of the orifice        [mm] is 50≦L≦150.

(5) A method of production of an ultralow carbon cast slab as set forthin (1) or (2), characterized in that

-   -   said immersion nozzle has a closed bottom cylindrical shape and    -   (i) two discharge ports are arranged at axially symmetric        positions to a cylinder at a bottom part of side walls of the        cylindrical shape and    -   (ii) a slit is provided connecting a cylinder bottom part and        bottom parts of the two discharge ports and opening to the        outside.

(6) A method of production of an ultralow carbon cast slab as set forthin (5), characterized in that, in said immersion nozzle,

-   -   (i) portions contiguous with the slit of the cylinder bottom        part are inclined upward toward the cylinder side walls    -   (ii) portions contiguous with the slit of discharge port bottom        parts are inclined upward toward discharge port side walls, and    -   (iii) there is substantially no step difference between the        surface forming the cylinder bottom part and the surfaces        forming the discharge port bottom parts.

(7) A method of production of an ultralow carbon cast slab as set forthin (6), characterized in that, in said immersion nozzle, an inclinationangle by which “portions contiguous with the slit” of the cylinderbottom part head toward the cylinder side walls and an inclination angleby which “portions contiguous with the slit” of the discharge portbottom parts head toward discharge port side walls are both at least 30°upward.

(8) A method of production of an ultralow carbon cast slab as set forthin any one of (5) to (7), characterized in that, in said immersionnozzle, portions of the top parts of the discharge ports contiguous withthe side walls of the cylinder are formed by curved surfaces smoothlycontiguous with the side walls of the cylinder.

(9) A method of production of an ultralow carbon cast slab as set forthin any one of (5) to (8), characterized in that said immersion nozzle isprovided with a rib connecting the two side surfaces of the slit.

(10) A method of production of an ultralow carbon cast slab as set forthin any one of (5) to (9), characterized in that, in said immersionnozzle, the slit has an opening width of 0.15 to 0.40 of a square rootof the cross-sectional area of the discharge port openings.

(11) A method of production of an ultralow carbon cast slab as set forthin any one of (1) to (10), characterized in that, in said immersionnozzle, the side surfaces and bottom part of the cylinder and part orall of the surfaces of the discharge ports and slit contiguous with themelt are formed by a material of any of carbon-less spinel, low-carbonspinel, magnesia graphite, zirconia graphite, and silica-less aluminagraphite.

According to the present invention, in continuous casting of ultralowcarbon steel, it is possible to prevent nozzle clogging and produce anultralow carbon cast slab free of surface flaws even if worked to athin-gauge steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an immersion nozzle having an orifice used inthe present invention.

FIG. 2 is a view showing an immersion nozzle with a circularcross-sectional shape and having an orifice in its inner bore.

FIG. 3 is a view showing an immersion nozzle with an ellipticalcross-sectional shape and having an orifice in its inner bore.

FIG. 4 is a view for explaining a preferable set position of an orifice.

FIG. 5 is a view showing an immersion nozzle with a circularcross-sectional shape and having two orifices.

FIG. 6 is a view showing an immersion nozzle with an ellipticalcross-sectional shape and having two orifices.

FIG. 7 is a view showing a generally used immersion nozzle with astraight shape with an inside diameter fixed from the top end to thebottom end of the inner bore.

FIG. 8 is a view showing an uneven distribution of the molten steel flowrate occurring when using the immersion nozzle shown in FIG. 7.

FIG. 9 is a view showing an uneven distribution of the molten steel flowrate and a state of a reverse rising flow occurring due to a strongdropping flow.

FIG. 10 is a view showing the relationship between an Ar gas flow rateand pinhole defects occurring in a cast slab.

FIG. 11 is a view showing the relationship between an Ar gas flow rateand surface flaws occurring in a steel sheet.

FIG. 12 gives views showing an immersion nozzle, used in the presentinvention, having a closed bottom cylindrical shape, having twodischarge ports at axially symmetric positions to the cylinder at thebottom of the side walls of the cylindrical shape, and provided with aslit connecting the bottom of the cylinder and the bottoms of the twodischarge ports and opening to the outside. (a) is a view showing an A-Across-section. (b) is a view showing a B-B cross-section. (c) is a viewshowing a C-C cross-section.

FIG. 13 gives views showing part of an immersion nozzle. (a) is a viewshowing an A-A cross-section. (b) is a view showing a B-B cross-section.(c) is a view showing a C-C cross-section. (d) is a view showing a D-Dcross-section.

FIG. 14 gives views showing part of another immersion nozzle. (a) is aview showing an A-A cross-section. (b) is a view showing a B-Bcross-section. (c) is a view showing a C-C cross-section. (d) is a viewshowing a D-D cross-section.

FIG. 15 gives views showing part of another immersion nozzle. (a) is aview showing an A-A cross-section. (b) is a view showing a B-Bcross-section. (c) is a view showing a C-C cross-section.

FIG. 16 gives views showing part of another immersion nozzle. (a) is aview showing an A-A cross-section. (b) is a view showing a B-Bcross-section. (c) is a view showing a C-C cross-section.

FIG. 17 gives views showing part of another immersion nozzle. (a) is aview showing an A-A cross-section. (b) is a view showing a B-Bcross-section. (c) is a view showing a C-C cross-section. (d) is a viewshowing a D-D cross-section.

FIG. 18 gives views another immersion nozzle used in the presentinvention. (a) is a view showing an A-A cross-section. (b) is a viewshowing a B-B cross-section. (c) is a view showing a C-C cross-section.

FIG. 19 gives views showing part of another immersion nozzle. (a) is aview showing an A-A cross-section. (b) is a view showing a B-Bcross-section. (c) is a view showing a C-C cross-section. (d) is a viewshowing a D-D cross-section.

FIG. 20 gives views showing part of another immersion nozzle. (a) is aview showing an A-A cross-section. (b) is a view showing a B-Bcross-section. (c) is a view showing a C-C cross-section. (d) is a viewshowing a D-D cross-section.

BEST MODE FOR CARRYING OUT THE INVENTION

To solve the problems in the prior art, the following three conditionsare major requirements:

Condition 1: Prevention of agglomeration and merger of inclusions toprevent formation of clusters.

Condition 2: Making position of secondary meniscus close to slidingnozzle as much as possible.

Condition 3: Using an immersion nozzle having an orifice in its innerbore and/or an immersion nozzle having a slit at its bottom.

First, as explained in the section of BACKGROUND ART, coarse clustersbecome causes of nozzle clogging and surface flaws in thin-gauge steelsheet, so Condition 1 has as its object the prevention of formation ofclusters.

Next, Condition 2 has as its object the suppression of unevendistribution of the flow rate occurring due to a dropping flow. Thecloser the position of the secondary meniscus to the position of thesliding nozzle, the shorter the dropping distance of the molten steel,so the more difficult the formation of a strong downward flow. Theuneven distribution of the flow rate due to the dropping flow issubstantially suppressed.

In the present invention, deoxidization by Ti-rare earth metals (Ce, La,Nd) enables the prevention of formation of clusters and prevention ofoccurrence of nozzle clogging at the casting stage.

For this reason, it is possible to greatly reduce the Ar gas flow ratefrom the Ar gas flow rate at the time of casting Al-killed molten steel.As a result, the secondary meniscus can be made close to the position ofthe sliding nozzle. Further, at the time of casting, if using animmersion nozzle having an orifice in its inner bore, the molten steelflow rate will be made even and uneven flow will be suppressed.

Condition 3 relating to the immersion nozzle used has as its object theelimination of the uneven distribution of the flow rate occurring due tothe sliding nozzle.

If setting the orifice in the inner bore of the immersion nozzle, themolten steel will stop once at the part above the orifice, so duringthis time, the uneven distribution of the flow rate will be eliminated.

If using an immersion nozzle having an orifice in its inner bore andhaving a slit at its bottom part, the uneven distribution of the flowrate will be eliminated, so powder or bubbles will not be carried intodeep positions of the unsolidified part of the molten steel. As aresult, the occurrence of surface flaws of the steel sheet due to powderor bubbles remaining in the cast slab (occurring when working the castslab into steel sheet) can be prevented.

When Condition 2 and Condition 3 are simultaneously satisfied, themolten steel remains for a relatively long time between the secondarymeniscus and the top end of the orifice, so the effect of rectificationof the flow by the orifice is improved more.

Note that the presence of occurrence of an uneven flow is usuallyevaluated using the difference in the coefficient of heat transfer atthe two short sides of the casting mold. The coefficient of heattransfer is an indicator showing the ease of transfer of heat throughthe wall surfaces. If the difference in the coefficient of heat transferof the two short sides of the casting mold is 250 J/(m²·s·K) or more,“asymmetric fluid motion of the molten steel”, that is, “uneven flow”,occurs as learned from experience.

The method of calculation of the coefficient of heat transfer is shownin formula (1) and formula (2). The coefficient of heat transfer h[J/(m²·s·K)] is calculated from the amount of heat removal q of thecasting mold [J/(m²·s)] and the temperature difference (T∞−TMD) betweenthe molten steel and the casting mold surface [K]. The amount of heatremoval of the casting mold is calculated from the change in temperature(tout−tin) before the casting mold cooling water passes over the copperplates and after it passes over it [K], the cooling water flow rate Qw[kg/s], the specific heat Cw of water [J/(kg·K)], and the copper platesurface area S [m²].

h=q/(T∞−TMD)  (1)

q=QwCw(tout−tin)/S  (2)

Below, the present invention will be explained in detail.

The present invention covers ultralow carbon steel. The upper limitvalue of the carbon concentration is not particularly limited, butthin-gauge steel sheet of ultralow carbon steel is used for steel sheetfor automobiles etc. subjected to severe working, so has to be providedwith superior workability. For this reason, the carbon concentration ispreferably 0.01 mass % or less. Note that the lower limit value of thecarbon concentration is not particularly defined.

In the present invention, in the secondary refining, the steel isdecarburized to a carbon concentration of 0.01 mass % or less, then Tiis added to the molten steel for deoxidation. The amount of addition ofthe Ti is preferably 0.04 mass % or more. If less than 0.04 mass %, thedeoxidation does not sufficiently occur and there is a high possibilityof dissolved oxygen remaining in the molten steel.

In secondary refining, when decarburizing steel to a carbonconcentration of 0.01 mass % or less, it is also possible to deoxidizethe steel by Al preliminarily before the decarburization in the refiningapparatus, for example, at the stage of refining by the converter. Inthis case, the Al concentration after deoxidation is made 0.01 mass % orless, preferably 0.008 mass % or less, more preferably 0.006 mass % orless.

If the Al concentration after deoxidation is 0.01 mass % or less, thedeoxidation product, that is, alumina, floats up to the surface of themolten steel in the interval until continuously casting the molten steeland can be removed, so the amount of alumina remaining in the moltensteel during casting becomes smaller and the problems of nozzle cloggingetc. do not occur.

Further, if the Al concentration after deoxidation is 0.008 mass % orless, the amount of alumina remaining in the molten steel during castingbecomes smaller, so this is preferable. Further, if the Al concentrationafter deoxidation is 0.006 mass % or less, the amount of aluminaremaining in the molten steel during casting becomes even smaller, sothis is more preferable.

On the other hand, the upper limit of the amount of addition of Ti isnot particularly defined. The Ti oxides produced by addition of Ti areharder to aggregate and merge than Al₂O₃-based inclusions, but easilystick to the refractory, so nozzle clogging is a concern.

Therefore, after deoxidizing the molten steel by Ti, at least one of Ce,La, and Nd is added. By this addition, the Ti oxides become hard toagglomerate and merge and are converted to spherical inclusions hard tostick to the refractory.

The total amount of addition of Ce, La, and Nd is preferably 0.001 mass% to 0.01 mass %. If the above total amount of addition is less than0.001 mass %, the modification of the Ti oxides becomes insufficient andspherical inclusions hard to agglomerate and merge become hard to form.Further, if over 0.01 mass %, the modification of the Ti oxide becomesexcessive, the Ti-based inclusions become heavier in specific gravityand become harder to float up, and the cleanliness of the molten steeldeteriorates.

Rare earth metals other than Ce, La, and Nd (for example, Pr, Sm, etc.)do not have effects of modification equal to those of Ce, La, and Nd, sofor modification of the Ti oxides, addition of one or more of Ce, La,and Nd is effective.

Here, FIG. 1 shows an immersion nozzle having an orifice used in thepresent invention. In the present invention, the portion of the innerbore 10 smaller in inside diameter than the top end of the inner bore isdefined as the “orifice 21”, while any portion with an inside diameterequal to the top end of the inner bore or with an inside diameter largerthan the same is defined as a “non-orifice part 21 z”. At the interfacesof the orifice 21 and the non-orifice parts 21 z, the boundary at theupstream side is referred to as the “orifice top end 21 a” and theboundary at the downstream side is referred to as the “orifice bottomend 21 b”.

As explained above, in the present invention, the Ar gas flow rate canbe greatly reduced from the time of the conventional Al deoxidation. Ifreducing the Ar gas flow rate, the secondary meniscus rises. Whenreaching 3 Nl/min or less, the secondary meniscus rises to a position ofabout 100 to 120 mm from the sliding nozzle.

If the secondary meniscus rises to the above position, almost no strongdownward flow occurs and the distance to the orifice top end can besufficiently secured, so it is possible to reliably eliminate unevendistribution of the molten steel flow rate. This fact was discovered bythe inventors. Note that as the value of the Ar gas flow rate in thepresent invention, it is possible to use a value measured using acommercially available flowmeter.

The smaller the Ar gas flow rate, the more the secondary meniscus rises,so the Ar gas flow rate is preferably 2 Nl/min or less, more preferablyless than 1 Nl/min.

If the Ti oxides are all suitably modified, almost no sticking of Tioxides to the immersion nozzle will occur, so the lower limit value ofthe Ar gas flow rate also includes 0 Nl/min.

Ar gas is generally blown in from one or more of a tundish upper nozzle,sliding nozzle, or immersion nozzle, but if in the range from thetundish upper nozzle to immersion nozzle discharge ports, the positionsof the locations where the Ar gas is blown in and the number of thoselocations may be freely selected.

In the immersion nozzle used in the present invention, to moreremarkably secure the effect of suppression of an uneven flow, there isa preferable range of orifice size.

As the cross-sectional shape of the immersion nozzle, normally acircular or elliptical shape is used. FIG. 2 shows an immersion nozzlewith a circular cross-sectional shape, while FIG. 3 shows an immersionnozzle with an elliptical cross-sectional shape.

When the cross-sectional shape is circular, the radius of the top end ofthe inner bore is defined as R [mm] and the smallest radius of theorifice is defined as r [mm]. On the other hand, when thecross-sectional shape is elliptical, the radius of the top end of theinner bore in the long axis direction is defined as A [mm] and thesmallest radius of the orifice in the long axis direction is defined asa [mm].

Here, for the orifice, the smallest radius is used because in thepresent invention, an orifice is defined as a “portion smaller in insidediameter than the top end of the inner bore”.

As the shape of the orifice, even a shape with an inside diameter notfixed from the top end to the bottom end of the orifice may beenvisioned, so the smallest radius was used to define it so that theinvention can be applied even to such a shape.

Next, the difference between the radius of the non-orifice parts and thesmallest radius of the orifice is defined as the “height of theorifice”. Normally, the radius of the non-orifice parts is equal to theradius of the top end of the inner bore, so the “height of the orifice”can be alternately referred to for the difference between the radius ofthe top end of the inner bore and the smallest radius of the orifice.

This being so, the “height of the orifice” is expressed by “R−r” whenthe immersion nozzle has a circular cross-sectional shape and “A−a” whenit has an elliptical one.

Further, the distance from the top end to the bottom end of the orificeis defined as the “length of the orifice” and labeled L [mm].

When the immersion nozzle has a circular cross-sectional shape, the“height of the orifice” preferably satisfies the relation “3≦R−r≦30”.The range of R−r<3 results in a small effect of the orifice in makingthe flow rate uniform and difficulty in suppressing the uneven flow,while the range of R−r>30 results in a remarkably larger flow rate ofthe molten steel passing through the orifice and the fluid motion in thecasting mold is easily detrimentally affected.

Next, the length L of the orifice [mm] preferably satisfies the relation50≦L≦150. With a range of L<50, before the flow rate is made uniform,the molten steel ends up passing through the orifice, so suppression ofan uneven flow is difficult. Further, with a range of L>150, the part ofthe small inside diameter becomes longer, so the flow rate of the moltensteel becomes remarkably greater and the fluid motion in the castingmold is easily detrimentally affected.

When the immersion nozzle has an elliptical cross-sectional shape, the“height of the orifice” preferably satisfies the relation “3≦A−a≦30”,while the length L of the orifice [mm] preferably satisfies the relation“50≦L≦150”. The reason is similar to the case where the cross-sectionalshape is circular.

In the immersion nozzle used in the present invention, the position ofthe orifice is not particularly limited. However, as shown in FIG. 4, ifthe top end U of the orifice is below the mid point M of the top end Tof the inner bore and the top ends B of the discharge ports, the moltensteel can be made to reliably stop and the uneven distribution of theflow rate can be easily eliminated, so this is preferred.

Further, the number of the orifices is preferably a plurality ratherthan one so the flow rectification effect becomes greater. However, ifthe number of orifices becomes greater, the parts where the flow rate ofthe molten steel is large will increase, so one or two is preferable.

FIG. 5 and FIG. 6 show immersion nozzles with two orifices.

When providing a plurality of orifices in an inner bore with a circularcross-sectional shape (see FIG. 5), the r and L of each i-th orificefrom the top end of the inner bore (respectively labeled as “ri” and“Li”) preferably satisfy the conditions 3≦R−ri≦30 and 50≦Li≦150.

Further, the top end of at least one of the orifices is preferably belowthe mid point of the top end of the inner bore and the top ends of thedischarge ports.

When providing a plurality of orifices in an inner bore with anelliptical cross-sectional shape (see FIG. 6), the a and L of each i-thorifice from the top end of the inner bore (respectively labeled as “ai”and “Li”) preferably satisfy the conditions 3≦A−ai≦10 and 50≦Li≦150.

Further, the top end of at least one of the orifices is preferably belowthe mid point of the top end of the inner bore and the top ends of thedischarge ports.

Here, other embodiments of the immersion nozzle used in the presentinvention will be explained.

FIG. 12 shows an embodiment of an immersion nozzle of anotherembodiment. The immersion nozzle 1 shown in FIG. 12 is a closed bottomcylindrical shape immersion nozzle. At the bottom part of thecylindrical side walls 5, two discharge ports 2 formed by discharge portside walls 7 and discharge port top parts 8 are formed symmetricallywith respect to the cylindrical axis, while at the cylinder bottom part4 and the bottom parts 6 of the discharge ports 2, a slit 3 formed byslit side walls 9 and opening to the outside is provided.

If providing a slit in an immersion nozzle, the discharge flow of themolten steel into the casting mold is more uniformly dispersed, theuneven flow is eliminated more, and further entrainment of powder ismore stably prevented, so this is more preferable. Here, there is asuitable relation between the opening width Ws of the slit 3 and thesquare root of the cross-sectional area Sz of the opening part 2 z of adischarge port 2 due to the reasons explained below.

First, if the slit opening width Ws/√(cross-sectional area Sz of openingof discharge port) is over 0.4 and the slit 3 becomes larger than thedischarge port 2, the flow rate of the molten steel passing through theslit 3 increases. Bubbles, inclusions, etc. in the molten steel arecarried to deep positions of the unsolidified parts of the molten steel,remain in the casting mold, and become causes of surface flaws at thetime of working into a thin-gauge steel sheet.

On the other hand, if the slit opening width Ws/√(cross-sectional areaSz of opening of discharge port) is less than 0.1, sticking ofinclusions at the slit side walls 9, abrasion of the slit side walls 9,etc. sometimes occur.

Due to the above reasons, the slit opening width Ws/√(cross-sectionalarea Sz of opening of discharge port) is suitably 0.15 to 0.4.

FIG. 13 shows an embodiment of a bottom part of the immersion nozzleshown in FIG. 12. In the immersion nozzle shown in FIG. 13, the portionsof the cylinder bottom part 4 contiguous with the slit 3 are inclinedtoward the cylindrical side walls 5 by an inclination angle θ₁, whilethe portions of the bottom parts 6 of the discharge ports contiguouswith the slit 3 are inclined toward the side walls 7 of the dischargeports by an inclination angle θ₂.

The inclination angles (θ₁ and θ₂) are preferably 30 to 60°. If theinclination angles are less than 30°, an eddy is formed in the immersionnozzle in some cases. If the inclination angles are over 60°, the topparts of the discharge ports approach the meniscus in the casting moldand the discharge flow easily entrains powder. Note that the inclinationangles are preferably 30° or more.

The inclination angle θ₁ of the cylinder bottom part 4 and theinclination angle θ₂ of the discharge port bottom parts 6 preferablymatch, but they do not necessarily have to match. The difference inangle when the inclination angle θ₁ and the inclination angle θ₂ do notmatch is preferably 10° or less. Further, if the surface of the cylinderbottom part 4 and the surfaces of the discharge port bottom parts 6 areformed to be on the same plane, the structure of the immersion nozzlebecomes simpler so this is preferable.

On the other hand, as shown in FIG. 14, the surface of the cylinderbottom part 4 and the surfaces of the discharge port bottom parts 6 maybe connected at the required angle if providing a step difference at theconnecting surfaces. As shown in FIG. 14, if forming the discharge portbottom parts 6 in directions descending toward the outer circumferenceof the immersion nozzle, it is possible to adjust the directions of thedischarge flows of the discharge ports 2 along with the inclinations ofthe discharge port top parts 8.

Regarding the step difference between the surface of the cylinder bottompart 4 and the surfaces of the discharge port bottom parts 6,substantially no step difference is enough. There is no need tocompletely eliminate the step differences. Here, “substantially no stepdifference” means the step difference is a step difference of an extentwhereby the continuity of the downward flow in the immersion nozzle 1and the discharge flows from the discharge ports 2 is not impaired.Specifically, the step difference should be of an extent of 5 mm orless.

As shown in FIG. 15, even if there is a slight step difference 12between the surface of the cylinder bottom part 4 and the surfaces ofthe discharge port bottom parts 6, the effect of the present inventionis not impaired.

The discharge ports 2 are preferably shaped, as shown in FIG. 12 to FIG.15, like baseball home plates, but as shown in FIG. 16 may also be arcshapes or curved shapes. The surface of the cylinder bottom part 4 mayalso be curved. In this case, the inclination angle θ₁ of the cylinderbottom part 4 and the inclination angle θ₂ of the discharge port bottomparts 6 should be made the average inclination angle of the vicinitywhere the cylinder bottom part 4 or discharge port bottom parts 6contact the slit.

The portions of the discharge port top parts 8 contacting the cylinderside walls 5, as shown in FIG. 17, are preferably formed by curvedsurfaces 11 smoothly contiguous with the cylinder side walls 5. Byforming the above portions by curved surfaces, the discharge flowseparating from the discharge port top parts 2 can be prevented andentrainment of powder into the discharge ports 2 can be suppressed.

The radius of curvature Rz of the curved surface 11 smoothly connectingthe discharge port top parts 8 and the cylinder side walls 5, as shownFIG. 17, is determined by the thickness t of the cylinder side walls 5and the discharge angle φ. If making φ small, a problem occurs in thestrength of the material forming the immersion nozzle, but Rz can bemade larger, so this is advantageous for prevention of divergence of thedischarge flow. For example, in the case where φ=45′ or so, the radiusof curvature Rz is preferably 50 to 100 mm.

Further, as the immersion nozzle used in the present invention, as shownin FIG. 18, one having an orifice 21 with an opening cross-sectionalarea smaller than the opening cross-sectional area of the cylinderbetween the top end of the nozzle and the discharge ports 2 ispreferable.

In an immersion nozzle having a slit at the cylinder bottom part, if theflow rate of the melt increases, the force of the flow of melt pushingthe slit wider increases. For this reason, as shown in FIG. 19, it ispreferable to provide the slit 3 with ribs 22 connecting the slit sidewalls 9. By providing the ribs 22, even if the force pushing the slit 3wider increases, deformation or breakage of the immersion nozzle 1 canbe prevented.

As the refractory used for the immersion nozzle, alumina graphite,alumina spinel, or another conventionally used refractory may be used.

However, depending on the ingredients of the molten steel, an aluminagraphite immersion nozzle will sometimes be corroded and dissolve duringcasting, so it is preferable to make the cylinder side walls andcylinder bottom part of the immersion nozzle or part or all of thesurfaces of the discharge ports and slit contiguous with the moltensteel out of any of carbon-less spinel, low-carbon spinel, magnesiagraphite, zirconia graphite, or silica-less alumina graphite (high meltloss resistance refractories).

FIG. 20 shows an immersion nozzle where the cylinder side walls 5 andcylinder bottom part 6 and all of the surfaces of the discharge ports 2and slit 3 contiguous with the molten steel are made of a high melt lossresistance refractory 23 and the other parts are made of an ordinaryrefractory 24.

In the present invention, there is extremely little sticking ofnonmetallic inclusions to the immersion nozzle and no nozzle clogging,so the surface of the cast slab is resistant to formation of surfacedefects due to cluster-like inclusions.

Further, the cast slab obtained by the present invention is free ofsurface flaws even if made into thin-gauge steel sheet by hot rolling,cold rolling, or other ordinary methods since the penetration of bubblesand powder causing surface flaws is suppressed at the time ofproduction.

Further, the present invention exhibits similar effects not only whenapplied to continuous casting of a slab of a usual thickness of 250 mm,but also in continuous casting of a thin slab with a casting moldthickness thinner than this, for example, 150 mm, so a cast slab of anextremely good quality can be obtained.

Examples 1

Below, examples and comparative examples (see Table 1) will be given toexplain the present invention using immersion nozzles of various shapes(see FIGS. 1 to 6).

TABLE 1 Max. difference of coefficient of heat Ar flow transfer atcasting Pinhole Deoxidizing rate Immersion nozzle Nozzle mold shortsides uneven defects Surface flaws Item agent (Nl/min) inner boresticking (J/(m² · s · K) flow (/m²) (/coil) Ex. 1-1 Ti and Ce—La 0Circular cross- None 200 None None 0.2 alloy section straight Insiderange of edge trim, so no problem Ex. 1-2 Ti and Ce—La 2.8 Circularcross- None 150 None None None alloy section with orifice Ex. 1-3 Ti andCe—La—Nd 0.5 Circular cross- None 100 None None None alloy section withorifice Ex. 1-4 Ti and Ce—La 0 Circular cross- None 50 None None Nonealloy section with orifice Ex. 1-5 Ti and Ce—La—Nd 0.5 Elliptical cross-None 100 None None None alloy section with orifice Ex. 1-6 Ti and Ce—La0.5 Circular cross- None 100 None None None alloy section with orificeEx. 1-7 Ti, Al, and 2.8 Circular cross- None 150 None None None Ce—La—Ndalloy section with orifice Comp. Al 7 Circular cross- Yes 300 Yes 15 10Ex. 1-1 section straight Comp. Al 2 Circular cross- Yes 330 Yes None 10Ex. 1-2 section with orifice Comp. Ti and Ce—La 7 Circular cross- None300 Yes 15 3 Ex. 1-3 alloy section straight Comp. Ti and Ce—La 4.5Circular cross- None 280 Yes 10 3 Ex. 1-4 alloy section straight Comp.Ti and Ce—La 3.5 Circular cross- None 260 Yes 5 1 Ex. 1-5 alloy sectionwith orifice

Example 1-1

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in a converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method without Ar(flow rate 0 Nl/min) to obtain a cast slab of a thickness of 250 mm anda width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, and the inner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 200 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab did not have any pinhole defects. Further,the surface of the cold rolled steel sheet had 0.2 surface flaw/coil.However, the surface flaws occurred at positions in the range of theedge trim, so did not become a problem in product quality.

Example 1-2

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2.8 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 150 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 1-3

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes; then Ce, La, and Nd were added by a Ce—La—Ndalloy of a mass ratio Ce/La=1.3 and La/Nd=3.5. The result was refluxedfor 3 minutes to produce molten steel having a Ti concentration of 0.03mass %, a total concentration of Ce, La, and Nd of 0.01 mass %, and a Ldconcentration/Nd concentration of 3.5.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 0.5 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 mm and an inside diameter of 85 mm. The materialof the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 100 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 1-4

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with no Ar(flow rate 0 Nl/min) to obtain a cast slab of a thickness of 250 mm anda width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 mm and inside diameter of 85 mm. The material ofthe inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 50 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 1-5

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce, La, and Nd were added by a Ce—La—Ndalloy of a mass ratio Ce/La=1.3 and La/Nd=3.5. The result was refluxedfor 3 minutes to produce molten steel having a Ti concentration of 0.03mass %, a total concentration of Ce, La, and Nd of 0.01 mass %, a Ceconcentration/La concentration of 1.3, and a Ld concentration/Ndconcentration of 3.5.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 0.5 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was an outer shape ofan ellipse with a long axis of 170 mm and a short axis of 120 mm and aninner shape of an ellipse with a long axis of 105 mm and a short axis of75 mm. The material of the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: A−a=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 100 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 1-6

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 0.5 Nl/min toobtain a cast slab of a thickness of 130 mm and a width of 1600 mm.

Here, as the thin slab continuous casting method, a casting mold of athickness of 150 mm or less was used for the casting. The cast slab washeld at a temperature of 1000 to 1200° C. by a holding furnace placed atthe downstream side of the machine end and hot rolled without cooling tonear ordinary temperature.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 70 mm. The material of the inner bore was aluminagraphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 100 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled to finally obtain acoil of cold rolled steel sheet of a thickness of 0.8-mm and a width of1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 1-7

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % and a concentration of Al of 0.005 mass % by refining inthe converter and treatment by a vacuum degassing apparatus, Ti wasadded for deoxidation. The result was refluxed for 6 minutes, then Ce,La, and Nd were added by a Ce—La—Nd alloy of a mass ratio Ce/La=1.3 andLa/Nd=3.5. The result was refluxed for 3 minutes to produce molten steelhaving a Ti concentration of 0.03 mass %, a total concentration of Ce,La, and Nd of 0.01 mass %, a Ce concentration/La concentration of 1.3,and a Ld concentration/Nd concentration of 3.5.

This molten steel was cast by the continuous casting method into a castslab of a thickness of 250 mm and a width of 1600 mm with a flow rate ofAr gas blown in from a tundish upper nozzle of 2.8 Nl/min.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 150 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Comparative Example 1-1

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Al was added for deoxidation. This was refluxed for5 minutes to produce molten steel with an Al concentration of 0.04 mass%.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 7 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite. The length from the top end of theinner bore to the top ends of the discharge ports was 590 mm, and theinner bore was straight in shape.

During casting, starting from when a cast mass of 150 tons is passed,the opening degree of the sliding nozzle of the tundish gradually becamelarger, so it was judged that inclusions had stuck to the immersionnozzle. The speed was reduced to secure the supply of molten steel tothe casting mold and the casting was completed.

At the casting mold short side copper plates, a maximum 300 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 15 pinhole defects/m², while the steelsheet had 10 surface flaws/coil.

Comparative Example 1-2

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Al was added for deoxidation. This was refluxed for5 minutes to produce molten steel with an Al concentration of 0.04 mass%.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite. The length from the top end of theinner bore to the top ends of the discharge ports was 590 mm, while theinner bore had a step difference of a height: R−r=5 mm and a length:L=90 mm (orifice). The top end of this step difference was at a position400 mm from the top end of the inner bore.

Soon after the start of casting, the opening degree of the slidingnozzle of the tundish gradually began to rise. At the point of time of acasting mass of 150 tons, even if making the opening degree full open,the supply of molten steel into the casting mold became insufficient. Atthis point of time, it was judged that the immersion nozzle was clogged.The casting was suspended in a state with a total of 130 tons of moltensteel left in the ladle and tundish (cast mass 170 tons).

During casting, at the casting mold short side copper plates, a maximum330 J/(m²·s·K) difference of coefficient of heat transfer occurred. Thiswas over the uneven flow judgment criteria of 250 J/(m²·s·K), so it wasjudged that uneven flow occurred.

Along with the progression of the clogged state of the immersion nozzle,it is believed that the effect of the step difference (orifice) waslost, an uneven flow rate distribution occurred, and an uneven flowresulted.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had no pinhole defects, but the steel sheethad 10 surface flaws/coil.

Comparative Example 1-3

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 7 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite. The length from the top end of theinner bore to the top ends of the discharge ports was 590 mm, and theinner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. However, at thecasting mold short side copper plates, a maximum 300 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 15 pinhole defects/m², while the steelsheet had 3 surface flaws/coil.

Comparative Example 1-4

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.2. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.2.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 4.5 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite. The length from the top end of theinner bore to the top ends of the discharge ports was 590 mm, and theinner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. However, at thecasting mold short side copper plates, a maximum 280 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 10 pinhole defects/m², while the steelsheet had 3 surface flaws/coil.

Comparative Example 1-5

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.2. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.2.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 3.5 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The cross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. The material ofthe inner bore was alumina graphite. The length from the top end of theinner bore to the top ends of the discharge ports was 590 mm, while theinner bore had a step difference of a height: R−r=5 mm, length: L=90 mm(orifice). The top end of this step difference was at a position 400 mmfrom the top end of the inner bore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. However, at thecasting mold short side copper plates, a maximum 260 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 5 pinhole defects/m², while the steelsheet had 1 surface flaw/coil.

Here, FIG. 10 shows the relation between the Ar gas flow rate and castslab pinhole defects based on Examples 1-1 to 1-6 and ComparativeExamples 1-3 to 1-5 (deoxidization by Ti-rare earth metals). In thefigure, the black dots show the case of use of an immersion nozzle withan inner bore of a straight shape, while the black squares show the caseof an immersion nozzle having an orifice in its inner bore.

Further, FIG. 11 shows the relation between the Ar gas flow rate andoccurrence of steel sheet surface flaws based on Examples 1-2 to 1-6 andComparative Examples 1-3 to 1-5 (deoxidization by Ti-rare earth metals).In the figure, the black dots show the case of use of an immersionnozzle with an inner bore of a straight shape, while the black squaresshow the case of an immersion nozzle having an orifice in its innerbore.

In the present invention, it is learned that if using an immersionnozzle having an orifice in its inner bore or an immersion nozzle withan inner bore of a straight shape and casting with an Ar gas flow rateof 3 Nl/min or less (region A in the figure), a good quality cast slabfree of both cast slab pinhole defects and steel sheet surface flaws canbe obtained.

Examples 2

Below, examples and comparative example (see Table 2) will be given toexplain the present invention using an immersion nozzle having a slit atits bottom (see FIG. 12 to FIG. 20).

TABLE 2 Max. difference of coefficient of heat Ar flow transfer atcasting Pinhole Deoxidizing rate Immersion nozzle Nozzle mold shortsides defect Surface flaws Item agent (Nl/min) inner bore sticking(J/(m² · s · K) Drift (/m²) (/coil) Ex. 2-1 Ti and Ce—La 2.8 Circularcross- None 100 None None None alloy section, straight slit width 15 mmθ₁, θ₂: 45° Ex. 2-2 Ti and Ce—La 0.5 Circular cross- None 80 None NoneNone alloy section, with orifice slit width 15 mm θ₁, θ₂: 45° Ex. 2-3 Tiand Ce—La 0 Circular cross- None 90 None None 0.2 alloy section, withorifice Inside range of edge slit width 15 mm trim, so no problem θ₁,θ₂: 45° Ex. 2-4 Ti and Ce—La 2.8 Circular cross- None 110 None None Nonealloy section, with orifice slit width 15 mm θ₁, θ₂: 45° Ex. 2-5 Ti, Aland 2.8 Elliptical cross- None 120 None None None Ce—La alloy section,straight slit width 15 mm θ₁, θ₂: 45° Ex. 2-6 Ti, Al and 0 Circularcross- None 50 None None 0.2 Ce—La alloy section, with orifice Insiderange of edge slit width 15 mm trim, so no problem θ₁, θ₂: 45° Comp. Al2.8 Circular cross- None 280 Yes 10 3 Ex. 2-1 section, straight slitwidth 15 mm θ₁, θ₂: 45° Comp. Al 2.8 Circular cross- None 260 Yes 5 1Ex. 2-2 section, straight slit width 15 mm θ₁, θ₂: 45° Comp. Al 2.8Circular cross- None 300 Yes 15 3 Ex. 2-3 section, straight slit width15 mm θ₁, θ₂: 45°

Example 2-1

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2.8 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 13. Thecross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 45°. Thematerial of the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, and the inner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 100 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 2-2

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 0.5 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 17 andFIG. 18. The cross-sectional shape of the immersion nozzle was a circlewith an outside diameter of 150 and an inside diameter of 85 mm, thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 45°. Theradius of curvature Rz of the curved surface 11 was 60 mm. The materialof the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 80 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 2-3

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 0 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 17 andFIG. 18. The cross-sectional shape of the immersion nozzle was a circlewith an outside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge port was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 45°. Thedischarge angle φ was 45°, while the radius of curvature Rz of thecurved surface 11 was 60 mm. The material of the inner bore was aluminagraphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 90 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab did not have any pinhole defects. Further,the surface of the cold rolled steel sheet had 0.2 surface flaw/coil.However, the surface flaws occurred at positions in the range of theedge trim, so did not become a problem in product quality.

Example 2-4

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Ti was added for deoxidation. The result wasrefluxed for 6 minutes, then Ce and La were added by a Ce—La alloy of amass ratio Ce/La=1.3. The result was refluxed for 3 minutes to producemolten steel having a Ti concentration of 0.03 mass %, a totalconcentration of Ce and La of 0.01 mass %, and a Ce concentration/Laconcentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2.8 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 19. Thecross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 45°. Thedischarge angle φ was 45°, while the radius of curvature Rz of thecurved surface 11 was 60 mm.

The vertex angle θ₃ of the rib was 30°, the width Wr of the bottomsurface of the rib was 15 mm, and the distance Dr between the side wallsof the rib was 85 mm. The material of the inner bore was aluminagraphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference is at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on theimmersion-nozzle and there was no problem with operability. Further, atthe casting mold short side copper plates, a maximum 110 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 2-5

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % and a concentration of Al of 0.005 mass % by refining inthe converter and treatment by a vacuum degassing apparatus, Ti wasadded for deoxidation. The result was refluxed for 6 minutes, then Ceand La were added by a Ce—La alloy of a mass ratio Ce/La=1.3. The resultwas refluxed for 3 minutes to produce molten steel having a Ticoncentration of 0.03 mass %, a total concentration of Ce and La of 0.01mass %, and a Ce concentration/La concentration of 1.3.

This molten steel was cast by the continuous casting method into a castslab of a thickness of 250 mm and a width of 1600 mm with a flow rate ofAr gas blown in from a tundish upper nozzle of 2.8 Nl/min.

The immersion nozzle used was the immersion nozzle shown in FIG. 13. Thecross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 45°. Thematerial of the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, and the inner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 120 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, there were neither pinhole defects in the cast slab norsurface flaws on the steel sheet.

Example 2-6

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % and a concentration of Al of 0.005 mass % by refining inthe converter and treatment by a vacuum degassing apparatus, Ti wasadded for deoxidation. The result was refluxed for 6 minutes, then Ceand La were added by a Ce—La alloy of a mass ratio Ce/La=1.3. The resultwas refluxed for 3 minutes to produce molten steel having a Ticoncentration of 0.03 mass %, a total concentration of Ce and La of 0.01mass %, and a Ce concentration/La concentration of 1.3.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 0 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 17 andFIG. 18. The cross-sectional shape of the immersion nozzle was a circlewith an outside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 45°. Thedischarge angle φ was 20°, while the radius of curvature Rz of thecurved surface 11 was 51 mm. The material of the inner bore was aluminagraphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, while the inner bore had a step differenceof a height: R−r=5 mm, length: L=90 mm (orifice). The top end of thisstep difference was at a position 400 mm from the top end of the innerbore.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. Further, at thecasting mold short side copper plates, a maximum 50 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was under theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow did not occur.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab did not have any pinhole defects. Further,the surface of the cold rolled steel sheet had 0.2 surface flaw/coil.However, the surface flaws occurred at positions in the range of theedge trim, so did not become a problem in product quality.

Comparative Example 2-1

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Al was added for deoxidation. The result wasrefluxed for 5 minutes to produce molten steel with an Al concentrationof 0.04 mass %.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2.8 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 13. Thecross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 15 mm, and the angles θ₁ and θ₂ were both 15°. Thematerial of the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, and the inner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. However, at thecasting mold short side copper plates, a maximum 280 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 10 pinhole defects/m², while the steelsheet had 3 surface flaws/coil.

Comparative Example 2-2

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Al was added for deoxidation. The result wasrefluxed for 5 minutes to produce molten steel with an Al concentrationof 0.04 mass %.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2.8 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used was the immersion nozzle shown in FIG. 13. Thecross-sectional shape of the immersion nozzle was a circle with anoutside diameter of 150 and an inside diameter of 85 mm. Thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 5 mm, and the angles θ₁ and θ₂ were both 45°. Thematerial of the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, and the inner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. However, at thecasting mold short side copper plates, a maximum 260 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a-coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 5 pinhole defects/m², while the steelsheet had 1 surface flaw/coil.

Comparative Example 2-3

To 300 tons of molten steel in a ladle given a concentration of carbonof 0.002 mass % by refining in the converter and treatment by a vacuumdegassing apparatus, Al was added for deoxidation. The result wasrefluxed for 5 minutes to produce molten steel with an Al concentrationof 0.04 masse.

This molten steel was cast by the continuous casting method with a flowrate of Ar gas blown in from a tundish upper nozzle of 2.8 Nl/min toobtain a cast slab of a thickness of 250 mm and a width of 1600 mm.

The immersion nozzle used when casting was the immersion nozzle shown inFIG. 13. The cross-sectional shape of the immersion nozzle was a circlewith an outside diameter of 150 and an inside diameter of 85 mm, thecross-sectional area of the opening of the discharge ports was 2829 mm²,the slit width was 25 mm, and the angles θ₁ and θ₂ were both 45°. Thematerial of the inner bore was alumina graphite.

The length from the top end of the inner bore to the top ends of thedischarge ports was 590 mm, and the inner bore was straight in shape.

During casting, there was no sticking of inclusions on the immersionnozzle and there was no problem with operability. However, at thecasting mold short side copper plates, a maximum 300 J/(m²·s·K)difference of coefficient of heat transfer occurred. This was over theuneven flow judgment criteria of 250 J/(m²·s·K), so it was judged thatuneven flow occurred.

The cast slab was cut to a length of 10000 mm to obtain 1 coil unit. Thesurface of this cast slab was examined by a CCD camera. The pinholedefects were evaluated by the number present per square meter cast slabsurface area.

Next, this cast slab was hot rolled and cold rolled by ordinary methodsto finally obtain a coil of cold rolled steel sheet of a thickness of0.8 mm and a width of 1600 mm.

The steel sheet quality was visually inspected on the inspection lineafter cold rolling and was evaluated by the number of surface flawsoccurring per coil.

As a result, the cast slab had 15 pinhole defects/m², while the steelsheet had 3 surface flaws/coil.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, it is possibleto prevent nozzle clogging and possible to produce an ultralow carboncast slab not giving rise to surface flaws even if processed intothin-gauge steel sheet. Accordingly, the present invention has a highapplicability in the steelmaking industry.

1. A method of production of an ultralow carbon cast slab characterizedby adding Ti to molten steel decarburized to a carbon concentration of0.01 mass % or less, further adding at least one of Ce, La, and Nd, andusing an immersion nozzle to inject the above molten steel from atundish to a casting mold for continuous casting while maintaining aflow rate of Ar gas blown from any location in a range from a tundishupper nozzle to discharge ports of said immersion nozzle at 3N1 (normalliter)/min or less.
 2. A method of production of an ultralow carbon castslab as set forth in claim 1, characterized in that said immersionnozzle has an orifice in its inner bore.
 3. A method of production of anultralow carbon cast slab as set forth in claim 2, characterized in thatsaid inner bore has a circular cross-sectional shape and (i) a relation3≦R−r≦30 stands between a radius R of a top end of the inner bore [mm]and a smallest radius r of the orifice [mm] and (ii) a length L from atop end to a bottom end of the orifice [mm] is 50≦L≦150.
 4. A method ofproduction of an ultralow carbon cast slab as set forth in claim 2,characterized in that said inner bore has an elliptical cross-sectionalshape and (i) a relation 3≦A−a≦30 stands between a radius A of along-axis direction of a top end of the inner bore [mm] and a smallestradius a of the orifice [mm] and (ii) a length L from a top end to abottom end of the orifice [mm] is 50≦L≦150.
 5. A method of production ofan ultralow carbon cast slab as set forth in claim 1, characterized inthat said immersion nozzle has a closed bottom cylindrical shape and (i)two discharge ports are arranged at axially symmetric positions to acylinder at a bottom part of side walls of the cylindrical shape and(ii) a slit is provided connecting a cylinder bottom part and bottomparts of the two discharge ports and opening to the outside.
 6. A methodof production of an ultralow carbon cast slab as set forth in claim 5,characterized in that, in said immersion nozzle, (i) portions contiguouswith the slit of the cylinder bottom part are inclined upward toward thecylinder side walls (ii) portions contiguous with the slit of dischargeport bottom parts are inclined upward toward discharge port side walls,and (iii) there is substantially no step difference between the surfaceforming the cylinder bottom part and the surfaces forming the dischargeport bottom parts.
 7. A method of production of an ultralow carbon castslab as set forth in claim 6, characterized in that, in said immersionnozzle, an inclination angle by which “portions contiguous with theslit” of the cylinder bottom part head toward the cylinder side wallsand an inclination angle by which “portions contiguous with the slit” ofthe discharge port bottom parts head toward discharge port side wallsare both at least 30° upward.
 8. A method of production of an ultralowcarbon cast slab as set forth in claim 5, characterized in that, in saidimmersion nozzle, portions of the top parts of the discharge portscontiguous with the side walls of the cylinder are formed by curvedsurfaces smoothly contiguous with the side walls of the cylinder.
 9. Amethod of production of an ultralow carbon cast slab as set forth inclaim 5, characterized in that said immersion nozzle is provided with arib connecting the two side surfaces of the slit.
 10. A method ofproduction of an ultralow carbon cast slab as set forth in claim 5,characterized in that, in said immersion nozzle, the slit has an openingwidth of 0.15 to 0.40 of a square root of the cross-sectional area ofthe discharge port openings.
 11. A method of production of an ultralowcarbon cast slab as set forth in claim 1, characterized in that, in saidimmersion nozzle, the side surfaces and bottom part of the cylinder andpart or all of the surfaces of the discharge ports and slit contiguouswith the melt are formed by a material of any of carbon-less spinel,low-carbon spinel, magnesia graphite, zirconia graphite, and silica-lessalumina graphite.